m6 – Open cluster


Lupe Rioné, Amateur astronomer (Spain)


Open cluster


Scorpio Constellation


1,600 light-years


12-20 light-years

Open clusters: Also called galactic clusters, because they are found within the galaxy, open clusters are groups of only tens or hundreds of young stars formed from the same nebula. They have a random, usually asymmetric, structure, and their stars are gravitationally bound to each other.

Do you want to know more about stellar clusters?

Previously we have navigated through the first globular clusters in Messier’s catalog. With M6, we will explore a new type of cluster: open clusters. These celestial bodies are clusters of hundreds or thousands of stars originating from the same molecular cloud. They are also known as galactic clusters, because they are scattered throughout the galaxy. They have a random and generally asymmetric structure, but the stars that make them up are gravitationally bound together. Unlike globular clusters, which are dense and massive, open clusters will end dispersed due to gravitational interactions between their stars or with other clusters. On the other hand, stars in open clusters are quite young, reaching a few hundred million years, while stars in globular clusters are the oldest objects in our galaxy, reaching more than 10 billion years old.

History and features

With an apparent magnitude of 4.2, the Italian astronomer Giovanni Battista Hodierna is considered the first to record the position of M6 in 1654, although some evidence suggest that first-century Alexandrian astronomer Claudio Ptolemy saw it with his naked eye. Then, in 1745, M6 was registered again by Jean Philippe de Cheseaux, and in May 1764 Charles Messier entered it in his catalog of celestial bodies and determined its position between the Sagittarius arch and the Scorpio’s tail.

In the 1960’s, American amateur astronomer Robert Burnhan, author of the masterful Celestial Handbook, referred to M6 as the butterfly cluster, due to the similarity of its stellar distribution to an open-winged butterfly. This beautiful cluster is formed by more than 200 stars and has a diameter of about 12-20 light years. It is estimated that its stars were formed about 100 million years ago, which means that while dinosaurs roamed the planet, M6 was just beginning to shine in the sky. For this reason, like other celestial bodies such as the Pleiades, most of the stars in M6 are blue giants of the main sequence spectral type B, with a magnitude of 7 or 8. Its brightest star is an orange supergiant called BM Scorpii (HD 1603791), a variable star that gives a touch of color to this beautiful cluster, and whose magnitude ranges from 5 to 7.7.

M6 open cluster. Credit: Marco Lorenzo

M6 cluster. Credit: Sergio Eguivar (Buenos Aires). Equipament: Camara QSI 583 WS, guide: Refractor Synta 70/400 with Starlight Spress LodeStar AstroArt 3.0

BM Scorpii has already left the main sequence and is burning elements heavier than hydrogen to support its gigantic force of gravity. As a result of its increased size, its surface temperature has dropped to about 3900 K and it has acquired a yellow-orange hue that contrasts with its blue giants. BM Scorpii’s changes in brightness affect the entire cluster, and for this reason, the magnitude of the entire cluster is linked to fluctuations in the magnitude of this large star. Experts estimate that the cluster will eventually break up because of the gravitational plays of its members.


This cluster is not difficult to observe, although it should be noted that it can be easily confused with the open cluster M7 due to its proximity and greater brightness. To find M6, just look for the Scorpion Constellation and locate its sting, which is a bright star we call Lambda. Towards the northeast (with a sufficiently dark sky) it can be seen as a bright patch with the naked eye. With a pair of binoculars, the stars in the cluster will have a similar brightness and their butterfly shape can be distinguished. With a telescope, many more individual stars and their colors can be seen, although the characteristic shape of the cluster will faint. Using a telescope larger than 200 mm, the enthusiasts can count the stars in this beautiful cluster.

Location of M6. Credit: ESO, IAU and Sky &Telescope

M6 Credit: Ole Nielsen.

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M10 – Globular Cluster | Cúmulo Globular

M10 is a very bright and star-rich cluster. Located in the constellation Ofiuco, it is estimated to be approximately 16,000 light-years from the center of our galaxy. An analysis of its radial velocity allows us to deduce that it is moving away from Earth at more than 271,440 km/h.

M9 – Globular Cluster | Cúmulo Globular

Like all globular clusters, M9 is a spectacle in itself, and one of the oldest and most intriguing objects in our galaxy. M9 stands out as one of the closest globular clusters to the center of our galaxy, only 5,500 light-years away.

M8 – Lagoon Nebula | Nebulosa de la Laguna

Like all emission nebulae, M8 or NGC 6523 is a gigantic HII region, that is, a spectacular cradle of stars. With a magnitude of 5.8, this incredible nebula boasts many large, hot stars, whose ultraviolet radiation sculpts the gases and cosmic dust, giving it its characteristic shape.

M7 – Open Cluster | Cúmulo Abierto

Also known as NGC 6475, M7 is an open cluster, that is, a group of gravitationally bound stars formed by the same molecular cloud. It is a very sparse cluster, located in a very rich region of the Milky Way.

M6 – Open Cluster | Cúmulo Abierto

This beautiful cluster comprises more than 200 stars and has a diameter of about 12-20 light-years. It is estimated that its stars were formed about 100 million years ago, which means that while dinosaurs roamed the planet, M6 was just beginning to shine in the sky.

M5 – Globular Cluster | Cúmulo Globular

The globular cluster M5 is probably one of the most interesting because it is one of the oldest, largest, and brightest clusters. It is located in the constellation of The Serpent (Serpens) at a distance of approximately 24,500 light-years.

M4 – Globular Cluster | Cúmulo Globular

Also called NGC 6121, M4 is the closest globular cluster to the Earth. M4 contains about 100,000 stars, and like most globular clusters, it is a very old structure, about 12-13 billion years old.

M3 – Globular Cluster | Cúmulo Globular

M3, also called NGC 5272, is one of the largest and most spectacular clusters that we can observe in our galaxy. In fact, M3 was the first original discovery by Charles Messier.

M2 – Globular Cluster | Cúmulo Globular

M2 is a globular star cluster and the second object in the Messier catalog. Also called NGC 7089, it was discovered in 1746 by French astronomer Jean-Dominique Maraldi.

M1 – Crab Nebula | Nebulosa del Cangrejo

M1 is the remnant of a supernova explosion that was observed by Chinese astronomers during the Song Dynasty in the 11th century.


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